It is a well known fact that heat transfer is quite difficult when the agents to be cooled have an inhomogeneous composition in cross-sectional direction of the heat exchange pipe. This is the case with agents composed of two different components or having two different phases such as gas and liquid. It is also well known that these agents have in most cases a wavy flow pattern or a ring-shaped flow pattern within the pipe. In the first case, the liquid phase flows in a lower part of the pipe and has a wavy free surface within the pipe, and the gas or vapor streams above this wavy surface. In the latter case, the liquid phase adheres in a ring from the inner wall of the pipe and encircles the gas or vapor streaming in the middle of the pipie. In both cases, the two phases don't stream together but they occupy two "flow channels" separate from each other.
This kind of agent is to be re-cooled in many fileds of the industry. In the case of heat pumps or of refrigerators, the agents to be cooled comprise a mixture of two components having different volatilities. These two phases differ not only in their states but in their concentrations, too. When the working agent having two phases is cooled, its temperature gets lower and, at the same time, dissolving and condensation occur. Since the two phases stream separately, they are not in a constant thermodynamic equilibrium and, therefore the component which is less volatile condensates quicker, the condensate recools quicker, and the component which is more volatile and forms the larger part of the gaseous phase dissolves later in the liquid phase. As a result, the temperature characteristics of the known heat exchange pipes dependent on the amount of heat transferred are quite disadvantageous. Thus, for a given thermodynamic process, a larger and more expensive heat/exchanger is required for providing the same thermo-dynamic coefficiency.
The same problems arise when the working agent in the pipe is warmed by a hotter outer agent such as water. Since the two components of the working agent get separated, the amount of heat which can be transferred is smaller than what is theoretically attainable.
In the operation of the known heat exchange pipes, a further disadvantageous effect can also be observed which arises with working agents having only one component. In a condenser, for example, the already condensed working agent forms a liquid phase which remains on the inner surface of the pipe wall, forming a resistance to the heat transfer between the non-condensed steam phase and the wall of the pipe.
It has also been found that the above-mentioned ring-shaped flow pattern is quite similar to that of the viscous liquids used as working elements in heat exchange pipes. In the first case, the composition of the agent itself is inhomogeneous, and in the latter, the physical conditions (temperature and viscosity) are inhomogeneous to a great extent.
It is a well known fact that, for example, oils which are used for the lubrication of the bearings of steam turbines or gas turbines and cooling thereof and which are cooled in heat exchangers to extract the heat arising from the mechanical heat losses from the bearings, are bad heat conductors and flow laminary in the pipes of the heat exchangers.
As a consequence of said properties, the heat transfer coefficient of the oils is low, having the disadvantageous consequence that cooling requires large and expensive heat exchangers.
The inferior heat transfer coefficient of laminar flowing oils with poor heat conductivity can be explained by the fact that the outer layer, having been cooled and flowing with a low velocity along the pipe surface, is acting as thermal insulation and hinders the path of the heat flux from the warmer oil towards the pipe wall. While the outer cooled oil is flowing forwards with a low velocity on the pipewall, forming a quasi denser layer on the pipewall, the warm oil flows in the middle of the pipe where it is hardly cooled. Heat is able to flow only by way of conductivity.
According to the practice developed earlier, longitudinally arranged inner ribs are used, which are parallel or substantially parallel to the longitudinal axis of the pipe. Essentially, the heat has to travel a shorter path in the cut-up cross section, accordingly resistance will be also less. However, the drawback of the ribs lies in that resistance, weight and therefore cost of production of the heat exchanger are also increased.
To lessen the effect of the aforementioned disadvantageous features, a heat exchange pipe for heat transfer from a medium in the pipe was proposed which includes spaced-apart baffle elements disposed within the pipe substantially perpendicularly to the longitudinal axis of the pipe and which have means for deflecting the outer layer of the medium away from the wall of the pipe. This is described in GB-PS 2 135 439.
While this solution can be regarded as the most developed one in the state of the art, a few disadvantageous features still deteriorate the efficiency of the heat exchangers. The known baffle element has a ring surface which is perpendicular to the wall which has to aid the deflecting action. But this ring surface causes a sharp break in the flow direction, which increases the flow resistance within the pipe and, at the same time, amplifies the tendency of the viscous liquid to by-pass the hindrance, i.e. the ring surface being in its flow path, without any substantial change in its laminar flow pattern in the boundary layer. Nevertheless, the known baffle element can only be used with said viscous liquids within certain speed and viscosity limits. It is not suitable for wavy flow patterns at all.